1. Field of the Invention
This invention relates to an optical disk cartridge in which an optical disk having a pair of recording faces formed on the opposite surfaces thereof is accommodated and also to an optical disk apparatus for recording and/or reproducing such optical disk.
2. Description of the Prior Art
An optical disk apparatus is conventionally known wherein an optical disk having a pair of recording faces formed on the opposite upper and lower surfaces thereof is placed in position on a turntable and the recording faces of the optical disk are recorded and/or reproduced by means of a pair of upper and lower optical heads.
In an optical disk apparatus of the type mentioned, a spot of a laser beam is irradiated upon a recording face of an optical disk by means of an objective lens of an optical head to effect recording (writing) or reproduction (reading) of information to or from the recording face. Accordingly, recording or reproduction of an optical disk is difficult if the distances of the upper and lower optical heads to the upper and lower recording faces, respectively, of the optical disk (here, such distances are equivalent to the focal lengths of spots of laser beams) are not set with a high degree of accuracy so that the spots may be focused on the recording faces of the optical disk.
Meanwhile, the thickness of a disk cartridge for such optical disk is specified in the national or international standards. However, since an optical disk is normally formed from a glass base plate, a plastic base plate or the like and has a different structure depending upon the a material used, it may have a different thickness depending upon the material used.
In an optical disk apparatus of the type described above, the distances of the upper and lower optical heads to the upper and lower recording faces, respectively, of the optical disk are set with reference to a height or vertical position of a turntable of a spindle motor on which the optical disk is received.
Accordingly, when the optical disk received on the turntable has a different thickness, the distance particularly of the upper optical head to the upper recording face of the optical disk is different from a specified distance, and consequently, recording or reproduction by the upper optical head may be difficult or impossible.
Thus, an optical disk apparatus has been developed wherein a head base for the upper optical head is adjusted upwardly or downwardly under the servoing control of a motor in order to optimize the distance of the upper optical head to the upper recording face of an optical disk in accordance with a dispersion in thickness of the optical disk.
Such solution of adjusting the head base of the optical head upwardly and downwardly in accordance with a dispersion in thickness of an optical disk under the servoing control of the motor to set the distance of the optical head to the recording face of the optical disk as in the conventional optical disk apparatus described just above, however, is disadvantageous in that the mechanical and electric (a servo circuit is required) constructions are very complicated and besides the speed upon such setting is low and the reliability in setting is low.